Multispeed synchronous dynamo-electric machine



Reissued May 2, 1933' UNITED STATES PATENT OFFICE PHILIP L. ALGER, OFSGHENECTADY, NEW YORK, ASSIGNIOR T GENERAL ELECTRIC COMPANY, ACORPORATION OF NEW YORK MULTISPEED SYNCHRONOUS DYNAMO-ELEGTRIG MACHINEOriginal No. 1,770,871, dated July 15, 1930, Serial No. 385,262, filedAugust 12, 1929. Application for reissue filed March 30, 1931.

My invention relates to dynamo electric machines and its principalobject is to provide a novel and simple circuit arrangement for theexicting windings of the magnetic field of synchronous dynamo electricmachines whereby they may be quickly changed from one number of poles toanother number of poles, thus giving two efficient operating speeds whenused either as a synchronous generator or a synchronous motor.

During the operation of water-wheel driven generators it frequentlyoccurs that the head or .quantity of water varies great- 'ly withdilferent seasons of the year, making the water-wheel operateinefiiciently part of the time because the most eflicient waterwheelspeed depends on the head of water available, and also on the quantityof flow. Previously this difficulty has been met by using specialwater-wheels with vanes that could be adjusted to give the maximumefficiency for the head of water used, but this complicated thewater-wheel construction. The maximum efficiency however can also 725 beobtained by varying the speed of the water-wheel when the head of watervaries. A water-wheel driven generator is usually connected 'to supply afixed frequency circuit and any deviation from its rated speed will varyits frequency unless provision is made for altering the number of itspoles. It is well known by those skilled in the art that with a fixednumber of'poles on a synchro- 7110115 generator its frequency will varyin direct proportion to its speed. Therefore it becomes desirable toobtain the maximum 'e-fficiency of the water-wheel at different heads ofwater and simultaneously maintain normal frequency and these desirableresults my invention accomplishes by changing the number of poles of'thegenerator.

Thus for example with a normal head of water the generator may beconnected for' 2N poles, N being any even number, and the generator willbe driven by thewater-wheel at its normal speed and it will give. itsnormal frequency. Vhen the head, or quantity of :flow -of water ismaterially reduced the generator may be reconnected for 3N poles and r"the generator will be driven by the water- Serial No. 526,506.

wheel at two-thirds of its normal speed, thereby maintaining mostefficient operation and normal frequency.

It is well known to those skilled in the art that with a fixed frequencythe speed of both synchronous and induction motors will vary in theinverse ratio at which their num; ber of poles are increased ordecreased. Certain types of large ships are normally propelled at theireconomical or so-ealled cruising speeds but for obvious reasons theymust have a maximumemergency speed well above the cruising speed. In thepast induction motors have been used for driving ships of this type andthe increase in speed was obtained by changing the motor pole number.The induction motor has an undesirable low power factor and efficiencyat the low or so-called cruising speeds and in addition has a greaterweight than a synchronous motor of corresponding .rating. It istherefore highly desirable to use two-speed synchronous motors bothbecause they have a higher eflicieney at the low or so-called cruisingspeeds and because their power factor can be adjusted to :unity for anysynchronous speed, thus giving two eflicient operating speeds. Thisdesirable result :my invention accomplishes by reducing the number ofpoles in a desired ratio when the speed is to be increased to itsmaximum and conversely increasing the number of poles in the same ratiowhen the speed is=t0 be decreased 130 normal. This speed change could beobtained in a single speed motor drivinga ship by changing the supplyfrequency, as for example by reducing the speed of "the turbine drivingthe supply generator. This procedure is objectionable however because itgreatly reduces the efficiency of the turbine. using twospeedsynchronous motors is evident.

My invention will be :best understood from the following descriptionconsidered inconnection with the accompanying drawings illustrating theinvention as applied for changing the speed in the ratio of 2 to 3,While the features of my invention which are believed to be novel andpatentableare pointed out ,in the claims appended hereto.

Throughout the text and drawings 28 indicates the normal speed of themachine; 35 indicates a speed 50% above normal; 3N indicates the numberof poles at the normal speed; 2N indicates the number of poles at 50%above the normal speed, N being an even number.

Fig. 1 represents the circuit connections of a revolving multi-polarfield element of a twospeed synchronous dynamo electric machineembodying my invention, while Fig. 2 represents a modification of thecircuit arrangement shown in Fig. 1. Fig. 1A represents the polarity ofthe field element when it is connected for 28 speed and 3N poles, whileFig.

1B represents the polarity of the field element when it is connected for3S speed and 2N poles, both Figs. 1A and 1B referring to the circuitarrangement shown in Fig. 1. Fig. 2A represents the polarity of thefield element when it is connected for 2S speed and 3N poles, while 2Brepresents the polarity of the field element when it is connected for 3Sspeed and 2N poles, both Figs. 2A and 2B referring to the circuitarrangement shown in Fig. 2. Figs. 3, 4, 5 and 6 represent flux curveswhich will be referred to in explaining my invention.

, Referring to Figs. 1 and 2, in both of which the field element isrepresented as developed on a plane surface, 10 represents the shaft ofa multipolar synchronous dynamo electric machine of the revolving poletype, 11, 12, 13,

. 14 and 15 represent the collector rings, 16 in Fig. 1, represents afive-pole double-throw reversing switch with one extraswitch prong 17',18 in Fig. 2 represents a two-pole, doublethrow reversing switch. A, B,C, D, E and F represent six salient poles which constitute a field unitand any number of such field units can be employed that will give therequired number of poles. For illustrative purposes I am showing onlyone field unit representing a field that can be changed from six polesto four poles or vice versa.

Fig. 1 shows the six salient poles connected in three separate circuitgroups. Group 1 consists ofexciting coils assembled on poles A and Dwhich are opposite to each other, groups 2 and 3 consisting of excitingcoils assembled on the adjacent pole pairs B and C and E and Frespectively, groups 2 and 3 being separated by the poles of group 1.The circuit group 1 consisting of every third pole piece shouldpreferably havethe ratio of the width of these pole pieces to the widthof the pole pieces of the other groups represented by any value from1.00 to .25 inclusive to obtain the best results. The width ratio of1.00 gives normal low speed characteristics with poor high speedcharacteristics, while a low width ratio gives good high speedcharacteristics at some sacrifice in low speed characteristics.

The two coils of each group are connected in series but are reversed inconnection so as to produce opposite polarities as shown in Fig. 1A, andtheir respective coil ends are connected to collector rings 11 and 12,12 and 13, 14 and 15, collector ring 12 functioning as a commonconnection for groups 1 and 2. The six-pole condition, is obtained byconnecting the three circuit groups in series or parallel to a suitabledirect current source through switch 16. The four-pole conditionisobtained by leaving group .1 open-circuited, or preferablyindependently short-circuited, leaving the connections to the directcurrent source of either group 2 or 3 unchanged, and reversing theconnections to the direct current source of the remaining group bothgroups remaining connected in series or parallel to the direct currentsource. For illustrative purposes I have shown group 1 shortcircuited,group 2' unchanged, and group 3 reversed. By tracing the current flow itis seen that with the switch 16 closed in the right position the threecircuit groups are connected in parallel to the direct current sourceand the connectionsare such that six poles of alternate polarity areproduced as shown in Fig. 1A, thus giving the 3N, 2S condition. Byclosing the switch 16 in the left position, group 1 is short circuited,group 2 is unchanged, group 3 is reversed, groups 2 and 3 remainingconnected in parallel to the direct current source, and the connectionsare such that four poles of alternate polarity are produced as shown inFig. 1B, thus giving the-2N, 3S condition. It is evident that under thefour-pole condition the two short circuited coils in group 1 do not useany direct current. It is well known to those skilled in the art thatwith the machine running in exact synchronism with the line frequencywhich is the predominating operating condition the two short-circuitedcoils do not cut any magnetic flux, hence a voltage will not be inducedin them and current will not flow in them. But a changed operatingcondition tending to make the machine hunt will cause the shortcircuited coils to cut magnetic flux, hence a voltage will be induced inthem and current will flow through them causing them to function asdamper windings and thus exert a steadying influence on the ma chine bypreventing speed oscillations and consequently help to keep the machinein synchronism with the line frequency. Any suitable switching apparatusmay be employed to obtain the six-pole and four-pole conditions andshort-circuit the group 1 coils in the four-pole condition. Forillustrative purposes I have shown in Fig. 1 a properly connectedfive-pole, double-throw, switch 16 for this purpose with one extraswitch prong 17 at the left position.

Referring to Fig. 2 which represents a modification of the circuitarrangement shown in Fig. 1, it is seen that we have the six salientpoles arranged in three separate groups. Group 1 consists of poles A andD which are opposite to each other but are with-- out exciting coils andhence are unable to produce magnetic flux. Groups 2 and 3 consist ofexciting coils on the adjacent poles B and C and E and F respectively,groups 2 and 3 being separated by the poles of group 1. The group 1,consisting of every third pole piece without exciting coils, shouldpreferably havethe ratio of the Width of these pole pieces to the widthof the pole pieces of the other groups represented by any value from1.00 to .25 inclusive, to obtain the best results, the choice depending011 the relative high and low speed characteristics desired. The twocoils of each circuit group are connected in series but reversed inconnection so as to produce opposite polarities as shown in Fig. 2A andtheir respective coil ends are connected to collector rings 12 and 13,and 14 and 15 respectively, collector ring 11 shown in Fig. 1 beingomitted as unnecessary. Both circuit groups are connected to a suitabledirect current source through switch 18. The six-pole condition isobtained by closing the switch 18 to the right and as can be seen bytracing the current flow the connections are such that the two-circuitgroups are connected in series to the direct current source and thereare produced two pairs of poles with opposite polarities in each pairand with like poles of each pair separated by the unexcited poles A andD. It is well known to those skilled in the art that when a metalsection susceptible to magnetization has like poles at the ends, theywill cause the center of such metal section to have an induced pole ofthe opposite polarity commonly known as a consequent pole. This is thesituation existing in the six-pole connection and therefore theunexcited salient poles A and D will become consequent poles, each onehaving an opposite polarity to the pair of like poles it separates, thusproducing six poles of alternate polarity as shown'in Fig. 2A, givingthe BN, 2S condition. To obtain the four-pole condition both circuitgroups remain connected in series but one of the circuit groups musthave its connections to the direct current source reversed. Forillustrative purposes I have shown the group comprising E and F thusreversible. By closing the switch 18 to the left and tracing the currentflow it can be seen that the two circuit groups are connected in seriesto the direct current source with the current reversed in poles E and Fthus producing four poles of alternate polarity as shown in Fig. 2B,giving the 2N, 3S condition. Any suitable s'witching apparatus may beemployed to obtain the six pole and four pole conditions, but forillustrative purposes I have shown in Fig. 2 a properly-connected doublepole, reversing switch 18 for this purpose.

The following theoretical discussion is well known to those skilled inthe art and when considered in connection with the drawings will be ofassistance in understanding the necessities for and the advantages of myinvention. In Fig. 1 if the ratio of the width of the pole pieces A andD to the width of the other pole pieces is represented by 1.00, thenunder the 3N pole condition there are sixpoles of alternate polaritywith a total peripheral pole arc of 1080 electrical de- 'grees asrepresented by the line '0 to the line 1080 in Fig. 3. Each pole pieceproduces a flux occupying 180 electrical degrees as shown for example bythe line 0 to the line 180 in Fig. 3 and the arcs between the center ofadjacent poles are 180 electrical degrees as shown for example by theline 90 to the line 270 and the arc between the centers of poles A and Dis 540 electrical degrees as shown for example by the line 270 to theline 810 in Fig. 3. Therefore we have in Fig. 3 a standard magnetizationand excitation with a sine wave flux of maximum strength resulting in astandard motor and the most eflicient operating condition 'because theefficiency is that of standard motor.

Fig. 4E represents the same pole pieces as in Fig. 3, but the fieldelement is now connected for 2N poles thereby giving four poles ofalternate polarity. The full line curve 20 in Fig. 4 represents the fluxproduced by the active exciting coils and the two horizontal lines ofcurve 20 show that zero magnetic flux is produced by the poles A and Dwhich are not excited in the 2N condition. The immediate impressionmight be that the irregular fiux wave as represented by curve 20 willproduce an inferior if not an entirely impractical machine. The advanceof the art A has however made it possible to design such machinesefiiciently principally by arrangement of the armature windings to opencircuit the voltages produced by the non-useful harmonics of themagnetic flux. The result ant characteristics of the machine are such aswould be produced by a machine having a magnetic flux represented by thedotted line curve 21 in Fig. 4 and which is almost a sine wave. As thereare four poles of alternate polarity therefore the total peripheral arcis'720 electrical degrees as represented by the lines 0 to the line 720in Fig. 1, and the arc of each pole of the resulting flux is 180electrical degrees as shown for example by the line 0 to the line 180 inFig. 4. As there are six pole pieces and the ratio of the width of thepole pieces A and D to the width of the other pole pieces is representedby 1.00 therefore the are between the centers of adjacent pole pieceswill be one-sixth of the total polar arc of 720 electrical degrees orequal to 1 20 electrical degrees as shown for example by the line 0 :tothe line 120 of Fig. 4 and the are between the centers of poles A and Dis 360 electrical degrees as shown for example by the line 180 to theline 540 in Fig. 4. In Fig. 4 there is astandard excitation with aresulting flux wave oflower strength than that of Fig. 3 andconsequently the efiiciency of the machine in Fig. 4 is lower than thatof Fig. 3. Also Fig. 4represents a'very poor magnetization of a machineembodying my invention because the ratio of the width of an unexcitedarea to an excited area is represented by 1.00 and yet I have found byactual and exhaustive tests that the machine was entirely normal andpractical except that its efficiency was lower than that of Fig. 3.

It is evident that to obtain the best possible flux condition as theratio of the width of the poles A and D to'the width of the other polesis reduced from 1.00 to .25 inclusive, then the ratio of the pole arcsoccupied by the flux of poles A and D to the pole arcs occupied by theflux of poles B, C, E and F should be correspondingly decreased. It ispertinent at this point .to show the arcs occupied by the flux of thepoles and the arcs between the centers of the poles with the ratios 1.00and .25 respectively. Under the 3N pole condition there are six poles ofalternate polarity with a total peripheral arc of 1080 electricaldegrees as represented by the line to the line 1080 in Fig. 5. With aratio of 1.00 the are occupied by the flux of every pole will be 180electrical degrees as shown in Fig. 3 and the arc between the centers ofadj acent poles will be 180 electrical degreesas shown in Fig. 3. With aratio of .25 the are occupied by the flux of poles A and D will each be.25 of the arc occupied by the flux of poles B, C, E and F and if W isassumed to be the are occupied by the flux of poles B, C, E and F thenthe total pole arc of the machine which is 1080 electrical degrees canbe represented by 4.5 W and therefore \V is 240 electrical degrees and.25 W is 60 electrical degrees. It is evident that the are between thecenters of poles B and C or E and F is 240 electrical degrees as shownfor example by the line 420 to the line 660 in Fig. 5 and the arebetween the centers of either pole A or D and its adjacent poles is 150electrical degrees as shown for example by the line 27 0 to the line 420in Fig. 5.

To illustrate a most striking example I have selected the ratio .25 inFig. 5. Inspection of Fig. 5 shows that the arcs occupied by the flux ofpoles A and D are each 60 electrical degrees as shown for example by theline 240 to the line 300 whereas the arcs occupied by the flux of polesB, C, E and F are each 240 electrical degrees as shown for example bythe line 0 to the line 240, thus maintaining the .25 ratio. Furtherinspection of Fig. 5 shows that the are between the center of eitherpole A or D and the center of its adjacent pole is 150 electricaldegrees as shown for example by the line 120 to the line 270, the arcsbetween the centers of either of the adjacent poles B and C or E and Fis 240 electrical degrees as shown for example by the line 420 totheline 660 and the arc betweenthe centers of the poles A and D is 540electrical degrees as shown for example by the line 270 to the line 810.The full line curve 22 in Fig. 5 represents the flux produced by theexciting coils and the small flux areas are those produced by the polesA and D. For reasons similar to those stated in the descriptionaccompanying Fig. 3 the resultant flux can be represented by the dottedline curve 23 in Fig. 5. Curve 23 shows six poles of alternate polaritywith a total peripheral arc of 1080 electrical degrees as shown by theline 0 to the line 1080 and an are for the flux of each pole of 180electrical degrees as shown for exampleby the line 0 to the line 180.Curve 23 is almost a sine wave but is of lower strength than curve: 19in Fig. 3 and consequently the efficiency is lowerthan that of Fig. 3.

Fig. 6 represents the same pole pieces as in F ig. 5 but the fieldelement is now connected for 2N poles, thereby giving four poles ofalternate polarity. The full line curve 24 in Fig. 6 represents the fluxproduced by the active exciting coils and the two horizontal lines ofcurve 24 show that Zero magnetic flux is produced by the poles A and Dwhich are not excited in the 2N condition. For reasons similar to thosestated in the description accompanying Fig. 3 the resultant flux can berepresented by the dotted line curve 25 in Fig. 6. Curve 25 shows fourpoles of alternate polarity with a total peripheral arc of 720electrical degrees, as shown by the line 0 to the line 720 in Fig. 6 andthe arc of each pole of the resulting flux is 180 electrical degrees asshown for example by the line 0 to the line 180 in Fig. 6. If W isassumed to be the are occupied by the flux of any of the poles B, C, Eor F, then .25 WV is the arc occupied by the flux of each of the poles Aand D, then the total pole arc of the machines which is720 electricaldegrees can be represented by 4.5 W and V is 160 and .25 \V iselectrical degrees Inspection of Fig. 6 shows that the arcs occupied bythe flux of poles A and D are each 40 electrical degrees as shown forexample by the line 160 to the line 200, whereas the arcs occupied bythe flux of poles B, C, E and F are each 160, electrical degrees asshown for example by the line 0 to the line 160. Further inspection ofFig. 5 shows that the are between the center of either pole A or D andthe center of its adjacent pole is 100 electrical degrees as shown forexample by the line to the line 180, the arcs between thecenters ofeither of the adjacent poles B andC or E and F is 160 electrical degreesas shown for example by the line 280 to the line 440 and the are betweenthe centers of the poles A and D is 360 electrical degrees as shown forexample by the line 180 to the line 540. Curve 25 in Fig. 6 is nearly asine wave but is of lower strength than curve 19 in Fig. 3 andconsequently the efii'ciency is lower than that of Fig. 3. 7

To recapitulate, the arcs occupied by the flux each of the poles A. andD will vary from a maximum of 180 to a minimum of 40 electrical degrees;the arcs. occupied by the flux of each of the poles B, C, E and F willvary from a maximum of 240 to a minimum of 120 electrical degrees; thearcs between the center of either pole A or D and the center of itsadjacent pole will vary from a maximum of 180 to a. minimum of 100electrical degrees; the arcs between the centers of the adjacent poles Band Cor E and F will vary from a maximum of 240 to a minimum of 120electrical degrees and the arcs between the centers of poles A and Dwill vary from a maximumof 540 to a minimum of 360 electrical degrees.

If flux curves are drawn for each of the conditions shown in Figs. 3, 4,5 and 6, but using only four exciting coils as explained in connectionwith Fig. 2, it will be seen that the arcs occupied by the flux of theVarious poles and the arcs between the centers of the various poles willbe the same as that of the corresponding condition using six excitingcoils. It is therefore unnecessary to illustrate the curves of thoseconditions using four exciting coils. Although all of the variationsdescribed give a lower efficiency than that of the standard motorrepresented in Fig. 3, yet it must be remembered that because of thehigher speed the 2N 3S conditions may give a greater actual horsepoweroutput than that of the 3N and 2S standard motor represented in Fig. 3.By the judicious selections of either four or six exciting coils and theratio that the width of every third pole piece that is in the same groupbears to the width of the other pole pieces, there can be obtained agreat variation in the efiiciencies at high and low speeds. Thus, forexample, if it is desired to have a large output at high speed and onlya relatively very small output at low speed, then the efliciency at highspeed should be as high as possible even if obtained at a sacrificeofthe low speed efliciency and especially is this true if the machine isto run at high speed the greaterpart of the time. Conversely, if only asmall output is desired at high speed, then the efficiency at low speedshould be as high as possible even if obtained at sacrifice of the highspeed efiiciency and especially is this true if the machine is to run atlow speed the greater part of the time. Thus if the ratio of the widthof every third pole piece that is in the same group to the width of theother pole pieces is represented by 1.00 the low speed is very efiioientand the high speed efficiency is lower as stated in the descriptionsaccompanying Figs. 3 and 4. On the other hand, as the ratio of the widthof every third pole piece in the same group to the Width of the otherpole pieces is being decreased from 1.00 to .25 the high speedefficiency increases and the low speed efficiency decreases as stated inthe descriptions accompanying Figs. 5 and 6. The following is anoutstandingexample of th e commercial applications in which theflexibility of my invention is very useful. Certain types of large shipsare normally propelled at their economical or so-called cruising speedsbut for obvious reasons they must have a maximum emergency speed wellabove the cruising speeds. But the power required to drive a screwpropelled ship increases approximately as the cube of the speed. But asthe ship runs at the low speed the greater part of the time thereforethe motor driving it should be designed with a high efliciency at thelow speed, whereas if the ship is to run at high speed most of the timethe motor efliciency should be high at the high speed. Another exampleis a centrifugal pump where the power required to drive it alsoincreases as the cube ofthe speed.

-I n a copending application, Serial No. 372,765 Pa tent No. 1,752,871,April 1, 1930, Robert W. Wieseman and assigned to the assignees ofthepresent invention, there is disclosed and broadly claimed a novel methodfor changing the number of poles in a synchronous dynamo electricmachine in a 2 to 3 ratio and vice versa. Briefly described thisinvention consists of 4N large salient poles and 4N small salient poles,N being any even number. The small salient poles are arranged in pairsand every pole piece has an exciting coil. The six-pole condition isobtained by so connecting all the exciting coils that every pair ofsmall salient poles acts as a single pole, whereas the four=polecondition is obtained by so connecting all the exciting coils that everysmall salient pole has the polarity of its adjacent large salient pole.It can be seen that Mr. W1esemans invention uses 8 pole pieces and 8exciting coils per field unit under both the 6- pole and l-polecondition, while with my invention for obtaining this 2 to 3 ratio I useonly 6 pole pieces and 6 exciting coils at the most per field unit. Mr.Wiesemans invention probably gives the most efliclent des1gn in certaincases while my invention has the flexibility of obtaining any relativeeflicien cy at high and lower speeds and thus both inventions have theirsphere of usefulness for this particular speed change ratio.

The stator windings of the machinehave not been shown but it will beunderstood that in this case they are wound so as to be ca pable ofconnectng for either 2N or 3N poles, N being any even number. This canbe accomplished by any of the methods used in induction motors as forexample the one disclosed in United States Patent 841,609, granted toErnst F.W'.Alexanderson. When a machine embodying my invention isstarted as a motor the field circuit will be open or short-circuitedthrough a resistance or reactance and the stator windings will normallybe connected for 3N poles. When the machine reaches the speedcorresponding to this connection the field windings may be energizedforSN poles if it is desired to operate at this condition. If it isdesired 'to operate at the 2N pole condition the field circuit i leftunenergized and the stator windings are changed over by an appropriatepole changing switch to 2N poles and when the machine reaches the speedcorresponding to this connection the field may be energized for the 2Npole condition. It is evident that during both starting conditions witheither the three-circuit or thetwo-circuit connection the inducedpotential across the end connections of each circuit group of the fieldwill be no more than one-half of what it would be if all the groups wereconnected in series and this is an evident advantage.

It is evident that the ratio of the width of every third pole piececomprising one group to the width of the other pole pieces may even berepresented by a value somewhat above 1.00 without departing from thespirit and scope of my invention.

lVhile I have described my invention in connection with a machine of therevolving pole type with a certain number of poles it is evident that myinvention is equally applicable to a machine with a stationary field ora different number of poles and therefore I do'not wish to limit myinvention to the type herein described. Also, while I have hereindescribed my invention in connection with its use in water wheelgenerators or as synchronous motors for large ships, it is evident thatthe uses described were only illustrative and that my invention isequally applicable foruse in any synchronous generator or synchronousmotor whenever it is desired to change the number of poles and thereforeI do not wish to limit my invention to the uses herein described,

In accordance with the provisions of the patent statutes, I havedescribed the principles of operation of my invention together with theapparatus'which I now consider to represent the best embodiment thereof,but I desire to have it understood that such other modifications as fallfairly within the true spirit and scope of my invention are intended tobe included within the scope of the appended claims.

What I claim as new and desire tosecure by Letters Patent of the UnitedStates, is:

1. In adynamo electric machine, a field element therefor, consisting ofone or more field units, each unit consisting of six pole pieces withexciting coils on these pole pieces connected in three separate circuitconnecting groups with the ratio of the width of every third pole piececomprising one group to the width of the other pole pieces representedby any value from 1.00 to .25 inclusive, means for energizing theexcitingcoils, and means for disconnecting one of the circuit groupsfrom the direct current source and reversing the current in one of theother circuit groups thereby changing the field element from 3N poles ofalternate polarity to 2\ poles of alternate polarity and Vice versa, Nbeing any even number.

2. In a synchronous dynamo electric machine of a type having windingsthat may be connected for either 2N poles or 3N poles, the said machinehaving a field element consisting of one or more field units, each unitconsisting of six pole pieces with exciting coils on the pole piecesconnected in three separate circuit connecting groups, two of thesecircuit groups consisting each of a pair of adjacent poles, said pairsof poles having approximately equal pole arcs between the centers oftheir pole pieces and each of said pole arcs having a value lyingbetween approximately 240 and 180 electrical degrees inclusiveconsidering the 3N pole connection and between approximately 160 and 120elec trical degrees inclusive considering the 2N pole connection, thethird circuit group consisting of a pair of pole pieces with the ratioof their width to the width of the other pole pieces represented by anyvalue from 1.00 to .25, inclusive, and the pole pieces of said thirdcircuit group having a pole arc between their centers of approximately540 electrical degrees considering the 3N pole connection and 360electrical degrees considering the 2N pole connection, means forenergizing the exciting coils, and means for disconnecting the lastmentioned group from the direct current source and reversing the currentin one of the other groups thereby changing the field elcment from 3Npoles of alternate polarity to 2N poles of alternate polarity and viceversa, N being any even number.

3. A field element for a dynamo electric machine consisting of one ormore field units, each unit consisting of six pole pieces with excitingcoils on these pole pieces connecting in three separate circuitconnecting groups withthe ratio of the width of every third pole piececomprising one group to the Width of the other pole pieces representedby any value from 1.00 to .25, inclusive, means for energizing theexciting coils, and means for disconnecting one of the circuit groupsfrom the direct current source and reversing the current in one of theother circuit groups, thereby changing the field element from 3N polesof alternate polarity in which every pole piece is magnetized by itsexciting coil to 2N poles of alternate polarity l in which N pairs ofadjacent poles are magnetized by their exciting coils with the unexcitedpoles approximately midway between the pairs of adjacent excited polesand with a pole are between the centers of the unexcited poles ofapproximately 360 electrical degrees, and vice versa, N being any evennumber.

4. In a dynamo electric-machine, a field element therefor consisting ofone or more field units, each unit consisting of six pole pieces withexciting coils on at least two pairs of pole pieces connected in atleast two separate circuit connecting groups, the remaining pole pieceshaving a ratio of their width to the width of the other pole piecesrepre sented by any value from 1.00 to .25 inclusive, means forenergizing the exciting coils, and means for reversing the current inone of the groups thereby changing the field element from 3N poles ofalternate polarity to 2N poles of alternate polarity and vice versa, Nbeing any even number.

5. In a synchronous dynamo electric machine of a type having windingsthat may be connected for either 2N or 3N poles, the said machine havinga field element consisting of one or more field units, each unitconsisting of six pole pieces with exciting coils on at least two pairsof pole pieces connected in at least two separate circuit connectinggroups, each group consisting of a pair of adjacent poles, said pairs ofpoles having approximately equal pole arcs between the centers of theirpole pieces and each of said pole arcs having a value lying betweenapproximately 240 and 180 electrical degrees inclusive considering the3N pole connection and between approximately 160 and 120 electricaldegrees inclusive considering the 2N pole connection, the remaining polepieces having a ratio or" their width to the width of the other polepieces represented by any value from 1.00 to .25 inclusive, the saidremaining pole pieces being situated approximately midway between thepairs of adjacent poles, the pole are between said remaining pole piecesbeing approximately 540 electrical degrees considering the 3N connectionand approximately 360 electrical degrees considering the 2N connection,means for energizing the exciting coils, and means for reversing thecurrent in one of the pairs of adjacent poles thereby changing the fieldelement from 3N poles of alternate polarity to EN poles of alternatepolarity and vice versa, N being any even number.

6. A field element for a dynamo electric machine consisting of one ormore field units, each unit consisting of six pole pieces with excitingcoils on at least two pairs of pole pieces connected in at least twoseparate circuit connecting groups, the remaining pole pieces having aratio of their width to the width of the other pole pieces representedby any value from 1.00 to .25 inclusive, means for energizing theexciting coils, and means for reversing the current in one of the groupsthereby changing the field element from 3N poles of alternate polarityin which at least two pairs of poles are magnetized by their excitingcoils to 2N poles of alternate polarity in which N pairs of adjacentpoles are magnetized by their exciting coils with the unexcited polesapproximately midway between the pairs of adjacent excited poles andwith a pole are between the centers of the unexcited poles ofapproximately 360 electrical degrees and vice versa, N "being any evennumber.

7. A field element for a dynamo electricmachine consisting of one ormore field units, each unit consisting of six pole pieces, excitingcoils on at least two pairs of pole pieces connected in at least twoseparate circuit connecting groups, means for energizing the excitingcoils, and means for reversing the current in'one of the groups therebychanging the field-element from 3N poles of alternate polarity to 2Npoles of alternate po- 1loarity and vice versa, N being any even num- 8.In a synchronous dynamo electric machine ot a type having windings thatmay be connected for either 2N or 3N poles, the said machine having afield element consisting of one or more field units, each unitconsisting of six pole pieces, exciting coils on at least two pairs ofpole pieces connected in at least two separate circuit connectinggroupsnneans for energizing the exciting coils, and means for reversingthe current in one of the groups thereby changing the field element from3N poles of alternate polarity in which at least two pairs of poles aremagnetized by their exciting coils to 2N of alternate polarity in whichN pairs of adjacent poles are magnetized by their exciting coils withthe unexcited poles approximately midway between the pairs of adjacentexcited poles, N being any even number.

9. In combination, a field element for a dynamo electric machinecomprising one or more units, each unit consisting of an even number ofpole pieces with an exciting coil surrounding each pole piece, means forconnecting said exciting coils into threecircuit connecting groups, oneof said groups consisting of two exciting coils surrounding two polepieces that are separated by one half of the remaining pole pieces, asecond of said groups consisting of the exciting coils surrounding saidone half of the remaining pole pieces, and the third of said groupsconsisting of the exciting coils surrounding the other half of theremaining pole pieces, and switching means for selectively connectingsaid three groups of exciting coils to a source of direct current toproduce alternate magnetic poles in all the pole pieces of said unit orconnecting only the second and third groups of exciting coils to thedirect current source to produce alternate magnetic poles in the polepieces corresponding to these groups.

10. In combination, afield element for a dynamo electric machinecomprising one or more u its, each unit consisting of an even number ofpole pieces with an exciting coil surrounding all except two of the polepieces, said twopole piecesbeing separated by one half of the remainingpole pieces, means for connecting the exciting coils surrounding saidone half of the remaining pole pieces into one group, means forconnecting the exciting coils surrounding the other half of theremaining pole pieces into a second group, and switching means forselectively connecting the two groups of exciting coils to a source ofdirect current to produce alternate magnetic poles in all the polepieces of said unit or connecting the two groups of exciting coils to asource of direct current to produce alternate magnetic poles in the polepieces corresponding to these groups.

In witness whereof, I have hereunto set hand.

- PHILIP L. ALGER.

